Thin films of BaTiO_3 are prepared on silicon substrates by spin-coating processing. The grain sizes of crystalline are determined by AFM observation. The average diameter of grains is about 50nm in films thicker than 60nm. In the thinner films than 30nm the grain size decreases. Far-infrared absorption spectra are measured by FTIR spectrometer as a function of thickness. The frequencies of phonons observed in films thicker than 30nm shows good correspondence with those of bulk crystals. However, in thinner films of 10nm the profiles of the absorption lines are rather different. The intensities of the low frequency modes at 180cm^<-1>,250cm^<-1> and 510cm^<-1> decrease as the thickness of film decreases, whereas the component of 375cm^<-1> mode remains its intensity. This suggests the change of crystal structure below the grain size of 30nm.An infrared reflection measurement was performed at room temperature in hexagonal barium titanate to determine the eigen frequencies and oscillator
… More strengths of the polar optic phonons of A_<2u> symmetry, including the soft phonon. It is found by means of the factorized form fitting that a total quantity of the relevant TO-LO splitting is as large as those in perovskite BaTiO_3. and SrT : O_3. The fact of the large TO-LO splitting shows that the dynamical effective charge must be large. The values of TO-LO splitting of hex-BaTiO_3and perovskite BaTiO_3 are almost same. In the perovskite type the each unit cell contains one formula of BaTiO_3, whereas in the hexagonal type there exists 6 formulae in the unit cell. The average volume about one formula is 67x10^<-30> m^3 for the hex-BaTiO_3 and 64x10^<-30> m^3 for the perovskite BaTiO_3 The average volume and the value of light side of equation (3) is almost similar, so that it can be predict that the average dynamical effective charges of each ion of hex-BaTiO_3 should be almost same as those of perovskite BaTiO_3. As the result it is concluded that large TO-LO splitting of the hex-BaTiO_3dues to the large shift of the dynamical effective charge from the nominal valence charge resulting the valance bonding character and that the octahedra of Ti and oxygen has the ferroelectric instability essentially ialso in the case of hex-BaTiO_3. Therefor the tendency of exhibiting ferroelectric instability in ATiO_2 is considered not to be respective of the stacking way of TiO_6 octahedra but to be primarily governed by the existence of TiO_6 octahedra.さらに、この微視的理論がペロフスカイト型に留まらず金属と酸素原子が形作る八面体構造を基本単位とする結晶構造を有する強誘電体の構造相転移に広く適応できる可能性をしらべる目的で、TiO_6八面体構造の配列がペロフスカイト型とは異なる六方晶チタン酸バリウムに着目し、FTIRにより反射率を測定した。赤外反射測定スペクトルを誘電関数に4パラメータ形式を仮定してフィッテングした。各格子振動のTOおよびLOモードの振動数、減衰定数および振動子強度等振動子パラメータからTO-LO分裂について、2.56×10^6cm^<-2>とペロフスカイト型強誘電体と同様な極めて大きな値を得た。この巨大なTO-LO分裂から上記の微視的理論が遷移金属と酸素原子が形作る八面体構造を有する誘電体の構造相転移に共通に適応できる正当性を示す重要な結果を得た。 Less